Just had the same experience with my Duet 0.8.5
Turns out you have to upgrade the firmware in a specific order to make it work.
I had to hold the erase button on the duet, then the reset to get it to show up on USB so i could flash an old firmware to start over.
If you try this be warned you need to use Bossac.exe (command line tool) because for some inexplicable reason the version with a GUI doesn't work on any modern PC. If like me you have an older board don't rename the firmware file before uploading.
This is specific to the WiFi board but the steps order helped me solve the issue.https://duet3d.dozuki.com/Wiki/Installing_and_Updating_Firmware